i386/xen: advertise XEN_HVM_CPUID_UPCALL_VECTOR in CPUID

This will allow Linux guests (since v6.0) to use the per-vCPU upcall
vector delivered as MSI through the local APIC.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>